FIG. 1 shows a distributed network. The network comprises a number of devices 1. Each device can communicate wirelessly with the other devices that are in effective range of it. In this example the network is a mesh network. The devices can cooperate to propagate signals (data) between them. For example, if device 1a transmits a signal, that signal can be received by devices 1b and 1c which are within range of device 1a. Devices 1b and 1c can then relay the signal received from device 1a so that it can be received by device 1d which is out of range of device 1a. The coverage area of device 1a is illustrated at 2a and the coverage area of device 1c is illustrated at 2c. This method of communication allows devices to communicate even though they are out of direct range of each other. However, it often has the consequence that, the time taken for the data to propagate from an initiating device to another device can be longer than if the data were conveyed in a single hop. Thus latency may be increased over a network in which data is conveyed in a single hop. Furthermore, when the number of hops needed for the signal to reach its destination is unknown, or may vary depending on propagation conditions or timing considerations, then the latency from the source to the destination may be unpredictable.
One example of a situation in which the latency can be unpredictable arises when the devices do not receive continually. For example, if the devices receive on a ⅓ duty cycle then (ignoring other propagation issues) there is roughly a 33% chance that a packet signal transmitted by one device will be received by another device that is in range. Consequently, in order for a signal to propagate from device 1a to device 1d it may end up going via more than two hops, or there may be retransmissions of the signal at one or more stages in the propagation chain. These effects can increase latency and make it more unpredictable.
It may be that in addition to performing their communication function the devices are cooperating to provide some service. For example, device 1a may be integrated with a light switch 3 and devices 1b and 1d may be integrated with light fittings 4. The signal transmitted by device 1a may signify that the light fittings are to turn on or off. If the light fittings respond to the on/off signal immediately it is received at the respective device then spatially distributed light fittings will turn on or off at slightly different times. If the light fittings are installed along a visually apparent path this may result in an obvious ripple effect as the light fittings change state in turn along that path. If the light fittings are installed over an area then they may give the impression of fading one by one. These effects may be desirable in some environments. However, in other environments it may be desirable for all the light fittings to change state simultaneously so as to mimic the behaviour of light fittings controlled by a traditional wired light switch.
Similar considerations can arise when appliances other than light fittings are associated with the wireless devices.
If the wireless devices operate using a wireless protocol that imposes a common timebase at or below the network layer then that timebase may be used to help synchronise the operation of the associated appliances. However, it may be that the wireless protocol imposes no common timebase at or below the network layer: as, for example in Bluetooth Low Energy. Indeed, it may be desirable for the protocol to impose no common network layer timebase. In order for a wireless communication device to operate with a protocol that uses a network level timebase the device must maintain a local clock. Maintaining an accurate local clock consumes more energy, and so it is desirable to minimise the time when an accurate local clock is active, even if a lower accuracy clock might be running continuously. That is of little concern if the wireless communication device is connected to a mains electric supply, as might be expected if it is associated with a light fitting. However, it may be that the range of the wireless communication devices is insufficient for mains-powered devices to communicate directly with each other. In addition, mains access points may be sparsely distributed, prohibiting direct communication between nodes and thus requiring additional nodes to be deployed that use batteries as power sources. In some environments it may even be desirable for all the nodes to be are battery powered. Mains-powered devices may have to communicate via other low-powered devices that are in the vicinity. Using a protocol that does not impose a common network timebase, or at least an accurate common network timebase, can reduce the power consumed by those devices and make it more likely that signals can be conveyed in an adventitious way between the appliances.
There is a need for a way of synchronising the operation of appliances in a system such as the one described above.